A technique of supplying an unburned fuel gas to an intake pipe of an engine is used in the related art to improve fuel consumption of the engine. For example, Patent Literature 1 describes a device which temporarily traps a fuel gas generated in a fuel tank in a canister and supplies the trapped fuel gas to an intake pipe. The fuel gas is supplied from the canister to the intake pipe through a fuel gas pipe connected to the canister and the intake pipe. The fuel gas is purged from the canister by using a negative pressure which develops when combustion air flows through the intake pipe.
The device described in Patent Literature 1 diagnoses a fuel gas supply system including the canister. To be more specific, the device detects an internal pressure of the fuel tank and also diagnoses presence or absence of an abnormality in the supply system according to the detected internal pressure.